This invention relates generally to coal formation detection and more specifically to coal layer boundary detection using borehole telemetry apparatus. It is well known that coal in natural formations may contain significant amounts of methane, a natural gas. It is further well known that coal is usually found in nominally horizontal beds and that economically significant amounts of methane can be recovered by boring holes into the coal bed. Such bored holes are nominally horizontal and the coal beds are relatively thin in vertical extent. U.S. Pat. Nos. 6,280,000 and 6,425,448 describe examples of such drilling and show particular patterns of holes to drain methane from the coal formation. In the boring of such holes, some means is needed to steer the drilling progress so as to remain in the coal bed and, to the extent possible, bore a straight hole such that up and down variations in the borehole path are minimized.
Conventional or current boring, or drilling, operations use some sort of measure-while-drilling (MWD) apparatus. Such an apparatus generally includes inclination and direction sensors, various logging sensors to assist in determining that the borehole trajectory remains in the coal seam and a communication means to transmit data to the surface so that the necessary control operations to control the drill string path can be performed. Typical inclination sensors include accelerometers to sense the earth's gravity field. The most commonly used direction sensors are magnetometers to sense the earth's magnetic field although gyroscopic sensors may be used in some circumstances. Logging sensors may include conventional resistivity sensors based in the low-megahertz frequency range, total gamma ray sensors and focused gamma ray sensors. In current practice, the only sensors that can provide reliable information on whether or not the drilling apparatus is within or out of the coal seam in surrounding rock formation are the various gamma ray sensors. Theses sensors generally have a very short range, perhaps only a few inches, and thus the drill bit may already be out of the coal seam by the time that gamma ray sensors provide an indication of such a condition. Given this limitation, such boreholes may have considerable variation in inclination as the path of the drill bit is steered to remain in the coal seam. Further, conventional resistivity tools would increase the length of the bottom hole assembly at the bottom of the drill string and would increase the cost of drilling. While certain resistivity apparatus and methods are used to steer the drilling apparatus in order to maintain the borehole in a desired geological bed, none of these is similar to or has the advantages of the present invention described below.
There is a need for sensing means that can efficiently detect the boundary of the coal seam, at a considerably greater depth of investigation around the borehole and most desirably one that can provide some indication of the conditions out ahead of the bit so as to permit correction of the drill path with reduced variation in inclination. It is well known that the resistivity of the coal in a coal seam may lie in the range of 50 to 100 ohm-meters and the resistivity of the adjacent rock, above or below the coal seam may lie in a range 1 to 4 ohm-meters.
In the measure-while drilling (MWD) process for drilling into coal seams, the borehole telemetry technique of choice is the electric field technique that involves direct injection of electric current into the surrounding formation at a point below an insulating gap in the generally conducting steel drill string. This injected current flows out into the formation and develops a detectable electric voltage between a remote contact to the earth and the drill string at the surface of the earth. Examples of such apparatus are disclosed in U.S. Pat. Nos. 5,130,706, 5,883,516, 6,188,223 and 6,396,276. It has been observed experimentally, and confirmed analytically, that when the drill bit is in a coal seam the apparent driving-point impedance, defined as the ratio of the output voltage to the output current, seen at the output stage of an electric field borehole telemetry apparatus decreases as the drill bit below an insulating gap approaches the coal seam boundary and penetrates into an adjacent rock layer. Further, it has been observed experimentally and confirmed analytically that the received signal strength at the surface of the earth increases for the same approach to and penetration into an adjacent rock layer.